Variable Focusing Optic for Aspheric Surface Measurement
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Solution Overview
Problem
Existing optical measurement technologies face challenges in accurately measuring freeform surfaces due to limited measurement ranges and difficulties in shaping illumination wavefronts to match the complex curvatures of aspheric surfaces, leading to measurement errors and ambiguities.
Innovation Solution
The use of a variable focusing optic and a multi-axis drive platform to systematically aberrate illumination wavefronts, allowing them to approximate the local shape of aspheric surfaces, with the ability to adjust focal length and translate both the surface and the measuring arm to maintain conjugacy with the detector, enabling precise measurement of subapertures and assembly of profile maps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional spherical illumination wavefronts are used to measure freeform surfaces, then the measurement setup is simple, but the measurement range is limited and measurement errors increase when surfaces depart significantly from spherical form
Solution Approach 1:
The patent employs a deformable mirror that can dynamically change its surface shape to match the curvature of different freeform surfaces being measured. This dynamic adaptability allows the illumination wavefront to be continuously adjusted, enabling accurate measurements across a wide range of surface geometries without requiring multiple fixed optical configurations.
Solution Approach 2:
The system changes the physical parameter of the illumination wavefront shape by using a deformable mirror to alter its surface curvature. This parameter change allows the wavefront to adapt from spherical to various freeform shapes, resolving the contradiction between maintaining simple setup and achieving high measurement precision for diverse surface types.
2Length of stationary object
If the measurement range is expanded to cover larger departures from spherical form, then more surfaces can be measured, but the fringe spacing decreases and patterns become ambiguous
Solution Approach 1:
The deformable mirror dynamically adjusts the illumination wavefront shape to match the local curvature of the surface being measured. This dynamic adaptation ensures that the wavefront remains well-matched to the surface geometry across the entire measurement range, preventing fringe pattern ambiguity even when measuring surfaces with large departures from spherical form.
Solution Approach 2:
The system applies local quality by tailoring the illumination wavefront shape to match the specific local curvature characteristics of different regions of the freeform surface. This localized adaptation maintains optimal fringe spacing and pattern clarity across the entire measurement range, resolving the contradiction between expanded range and pattern readability.
3Adaptability or versatility
If different shaping optics are substituted to accommodate different freeform shapes, then various surface types can be measured, but the device complexity and calibration difficulty increase
Solution Approach 1:
The deformable mirror provides a single dynamic optical element that can continuously adapt its shape to match various freeform surface geometries. This eliminates the need to physically substitute different shaping optics for different surface types, reducing device complexity while maintaining high versatility for measuring diverse surface forms.
Solution Approach 2:
The deformable mirror serves as a universal optical element that can generate illumination wavefronts matching multiple different freeform surface types. This multi-functionality replaces the need for multiple specialized optics, simplifying the overall device while expanding the range of measurable surfaces.
4Measurement precision
If adjustable shaping optics are used to match wavefront shapes, then measurement accuracy can be maintained, but errors and ambiguities are introduced that are difficult to resolve
Solution Approach 1:
The deformable mirror is controlled by a computer system that automatically adjusts the mirror shape to match the intended freeform surface geometry. This self-adjusting capability eliminates the need for manual calibration and monitoring of shaping optics contributions, reducing human error and improving measurement reliability while maintaining high wavefront matching accuracy.
Solution Approach 2:
The system uses computer control to monitor and adjust the deformable mirror shape based on the specific freeform surface being measured. This feedback mechanism ensures accurate wavefront matching while automatically compensating for potential errors, thereby maintaining both measurement precision and reliability without requiring complex manual calibration procedures.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach expands the dynamic measurement range, reduces errors, and allows for accurate characterization of freeform surfaces by distinguishing low-frequency shape characteristics from mid-frequency profile characteristics, enhancing the precision and efficiency of optical measurements.
Implementation Method 1
shaping an illumination wavefront with a variable focusing optic
Implementation Method 2
collecting and measuring a reflected test wavefront from the freeform surface
Implementation Method 3
measuring instruments, particularly those that exploit the mechanism of interference
Data Source
AI summary
An optical measuring instrument for measuring aspheric surfaces includes an optical measuring arm and a multi-axis drive platform. The optical measuring arm provides for illuminating and imaging the aspheric surfaces. The multi-axis drive platform relatively moves the optical measuring arm with respect to the aspheric surfaces through a plurality of subaperture measurement positions. A focus of adjustable focusing optic is maintained at a nominal center of curvature of the aspheric surfaces. A variable optical aberrator adds aberration to an illumination wavefront to match the illumination wavefront to the intended local shape of the aspheric surface. Fitted low-frequency shape information is distinguished from a remainder of the local shape information yielding mid-frequency topographic measurements of the subapertures, which can be assembled to construct a profile measurement of the aspheric surface.